This invention relates to high voltage protection circuits for television receivers.
In television receivers, the ultor high voltage accelerating potential for beam current in many instances is derived from a voltage developed across a tertiary winding of a horizontal output transformer. In transistorized deflection circuits, the high voltage is a function of such factors as the regulated B+ operating voltage, the values of the resonant retrace circuit, and the internal impedance of the output transformer and the high voltage rectification and multiplication circuits. The magnitudes of the high voltage will vary as beam current is being drawn by the cathode ray tube, the high voltage increasing relatively sharply for beam current decrements at low beam current levels.
To protect against the generation of harmful X-radiation, receiver circuits may include a high voltage protection circuit. The protection circuit senses the high voltage with a voltage developed across a secondary winding of the horizontal output transformer and disables normal receiver operation should the high voltage increase beyond a predetermined limit, that is, beyond a trip voltage. Because an increase in beam current will produce an increase in X-radiation for a given high voltage, the high voltage limit for X-radiation safety decreases appreciably with increasing beam current. The protection circuit, being responsive only to high voltage, must therefore be sensitive to a relatively low trip voltage at high beam currents.
Certain television receiver circuits, such as beam limiter circuits, are designed to be responsive to high beam currents. Their function, however, is not to protect against generation of harmful X-radiation, but to protect cathode ray tube components against failure from beam overload conditions due to excess video drive, channel changing induced noise, and momentary arcing between the gun electrodes.
Beam limiters will begin to operate only at relatively large beam currents. The high voltage protection circuit must provide protection at all beam current levels. However, since the protection circuit is only voltage sensitive, its trip voltage changes relatively little with changes in beam current. It is possible for a particular television receiver to operate at low beam currents and at a high voltage which, though not producing any harmful X-radiation, will be greater than the trip voltage of the protection circuit. Normal televison receiver operation will be unnecessarily disabled. It is desirable, therefore, to design a high voltage protection circuit which will eliminate such nuisance tripping.